Travis: use gimme for Go 1.5

.travis.yml

@@ -1,6 +1,5 @@
 language: python
 
-# This have to be kept in sync with .travis.yml from Boulder
 go:
   - 1.5
 
@@ -9,9 +8,11 @@ services:
   - mysql
 
 # http://docs.travis-ci.com/user/ci-environment/#CI-environment-OS
+# gimme has to be kept in sync with Boulder's Go version setting in .travis.yml
 before_install:
   - travis_retry sudo ./bootstrap/ubuntu.sh
   - travis_retry sudo apt-get install --no-install-recommends nginx-light openssl
+  - 'eval "$(gimme 1.5)"'
 
 # using separate envs with different TOXENVs creates 4x1 Travis build
 # matrix, which allows us to clearly distinguish which component under